Drilling system and method

ABSTRACT

A closed-loop drilling system and method of drilling oil, gas, or geothermal wells is described, whereby through the control of the flow rates in and out of the wellbore, and by adjusting the pressure inside the wellbore by a pressure/flow control device installed on the return line, surface pressure being increased or decreased as required, this in turn decreasing or increasing downhole pressure, occurrence of kicks and fluid losses may be greatly minimized and quickly controlled. Through the method of the invention the elimination of the kick tolerance and tripping margin on the design of the well is made possible, since the pore and fracture pressure will be determined in real-time while drilling the well, and, therefore, nearly no safety margin is necessary when designing the well, reducing significantly the number of casing strings necessary. The inventive method can be called intelligent safe drilling since the response to influx or fluid loss is nearly immediate and so smoothly done that the drilling can go on without any break in the normal course of action. The new method is applicable to the whole wellbore from the first casing string with a BOP connection, and it can be implemented and adopted to any rig or drilling installation that uses the conventional method with very few exceptions and limitations. The new method is applicable to all types of wells, onshore, offshore, deepwater and ultra-deepwater, with huge safety improvement in difficult drilling scenarios.

FIELD OF THE INVENTION

[0001] The present invention deals with a closed-loop system fordrilling wells where a series of equipment, for the monitoring of theflow rates in and out of the well, as well as for adjusting the backpressure, allows the regulation of the out flow so that the in and outflows are constantly balanced at all times. A pressure containmentdevice keeps the well closed at all times. Since this provides a muchsafer operation, its application for exploratory wells will greatlyreduce the risk of blow-outs. In environments with narrow margin betweenthe pore and fracture pressure, it will create a step change compared toconventional drilling practice. In this context, applications in deepand ultra-deep water are included. A method for drilling, using saidsystem, is also disclosed. The drilling system and method are suited forall types of wells, onshore and offshore.

BACKGROUND INFORMATION

[0002] Drilling oil/gas/geothermal wells has been done in a similar wayfor decades. Basically, a drilling fluid with a density high enough tocounter balance the pressure of the fluids in the reservoir rock, isused inside the wellbore to avoid uncontrolled production of suchfluids. However, in many situations, it can happen that the bottomholepressure is reduced below the reservoir fluid pressure. At this moment,an influx of gas, oil, or water occurs, named a kick. If the kick isdetected in the early stages, it is relatively simple and safe tocirculate the invaded fluid out of the well. After the originalsituation is restored, the drilling activity can proceed. However, if,by any means, the detection of such a kick takes a long time, thesituation can become out of control leading to a blowout. According toSkalle, P. and Podio, A. L. in “Trends extracted from 800 Gulf Coastblow-outs during 1960-1996” IADC/SPE 39354, Dallas, Tex., March 1998,nearly 0.16% of the kicks lead to a blowout, due to several causes,including equipment failures and human errors.

[0003] On the other hand, if the wellbore pressure is excessively high,it overcomes the fracture strength of the rock. In this case loss ofdrilling fluid to the formation is observed, causing potential dangerdue to the reduction in hydrostatic head inside the wellbore. Thisreduction can lead to a subsequent kick.

[0004] In the traditional drilling practice, the well is open to theatmosphere, and the drilling fluid pressure (static pressure plusdynamic pressure when the fluid is circulating) at the bottom of thehole is the sole factor for preventing the formation fluids fromentering the well. This induced well pressure, which by default, isgreater than the reservoir pressure causes a lot of damage, i.e.,reduction of near wellbore permeability, through fluid loss to theformation, reducing the productivity of the reservoir in the majority ofcases.

[0005] In the last 10 years, a new drilling technique, underbalanceddrilling (UBD) is becoming more and more popular. This technique impliesa concomitant production of the reservoir fluids while drilling thewell. Special equipment has been developed to keep the well closed atall times, as the wellhead pressure in this case is not atmospheric, asin the traditional drilling method. Also, special separation equipmentmust be provided to properly separate the drilling fluid from the gas,and/or oil, and/or water and drilled cuttings.

[0006] The UBD technique has been developed initially to overcome severeproblems faced while drilling, such as massive loss of circulation,stuck pipe due to differential pressure when drilling depletedreservoirs, as well as to increase the rate of penetration. In manysituations, however, it will not be possible to drill a well in theunderbalanced mode, e.g., in regions where to keep the wellbore wallsstable a high pressure inside the wellbore is needed. In this case, ifthe wellbore pressure is reduced to low levels to allow production offluids the wall collapses and drilling cannot proceed.

[0007] Since among the most dangerous events while drillingconventionally is to take a kick, there have been several methods,equipment, procedures, and techniques documented to detect a kick asearly as possible. The easiest and most popular method is to compare theinjection flow rate to the return flow rate. Disregarding the drilledcuttings and any loss of fluid to the formation, the return flow rateshould be the same as the injected one. If there are any significantdiscrepancies, drilling is stopped to check if the well is flowing withthe mud pumps off. If the well is flowing, the next action to take is toclose the blow-out preventer equipment (BOP), check the pressuresdeveloped without circulation, and then circulate the kick out,adjusting the mud weight accordingly to prevent further influx.

[0008] This procedure takes time and increases the risk of blow-out, ifthe rig crew does not quickly suspect and react to the occurrence of akick. Procedure to shut-in the well can fail at some point, and the kickcan be suddenly out of control. In addition to the time spent to controlthe kicks and to adjust drilling parameters, the risk of a blow-out issignificant when drilling conventionally, with the well open to theatmosphere at all times.

[0009] Among the methods available to quickly detect a kick the mostrecent ones are presented by Hutchinson, M and Rezmer-Cooper, I. in“Using Downhole Annular Pressure Measurements to Anticipate DrillingProblems”, SPE 49114, SPE Annual Technical Conference and Exhibition,New Orleans, La., 27-30 September, 1998. Measurement of differentparameters, such as downhole annular pressure in conjunction withspecial control systems, adds more safety to the whole procedure. Thepaper discusses such important parameters as the influence of ECD(Equivalent Circulating Density, which is the hydrostatic pressure plusthe friction losses while circulating the fluid, converted to equivalentmud density at the bottom of the well) on the annular pressure. It isalso pointed out that if there is a tight margin between the porepressure and fracture gradients, then annular pressure data can be usedto make adjustments to mud weight. But, essentially, the drilling methodis the conventional one, with some more parameters being recorded andcontrolled. Sometimes, calculations with these parameters are necessaryto define the mud weight required to kill the well. However, annularpressure data recorded during kill operations have also revealed thatconventional killing procedures do not always succeed in keeping thebottomhole pressure constant.

[0010] Other publications deal with methods to circulate the kick out ofthe well. For example, U.S. Pat. No. 4,867,254 teaches a method of realtime control of fluid influxes into an oil well from an undergroundformation during drilling. The injection pressure p_(i) and returnpressure p_(r) and the flow rate Q of the drilling mud circulating inthe well are measured. From the pressure and flow rate values, the valueof the mass of gas M_(g) in the annulus is determined, and the changesin this value monitored in order to determine either a fresh gas entryinto the annulus or a drilling mud loss into the formation beingdrilled.

[0011] U.S. Pat. No. 5,080,182 teaches a method of real time analysisand control of a fluid influx from an underground formation into awellbore being drilled with a drill string while drilling andcirculating from the surface down to the bottom of the hole into thedrill string and flowing back to the surface in the annulus definedbetween the wall of the wellbore and the drill string, the methodcomprising the steps of shutting-in the well, when the influx isdetected; measuring the inlet pressure P_(i) or outlet pressure P_(o) ofthe drilling mud as a function of time at the surface; determining fromthe increase of the mud pressure measurement, the time t_(c)corresponding to the minimum gradient in the increase of the mudpressure and controlling the well from the time t_(c).

[0012] It is observed that in all the cited literature where thedrilling method is the conventional one, the shut-in procedure iscarried out in the same way. That is, literature methods are directed tothe detection and correction of a problem (the kick), while there are noknown methods directed to eliminating said problem, by changing orimproving the conventional method of drilling wells.

[0013] Thus, according to drilling methods cited in the literature, thekicks are merely controlled. On the contrary, the present applicationrelates to a new concept of drilling whereby a method and correspondinginstrumentation allows that kicks may be detected early and controlledmuch quicker and safer or even eliminated/mitigated than instate-of-the-art methods.

[0014] Further, it should be noted that the present method operates withthe well closed at all times. That is why it can be said that themethod, herein disclosed and claimed, is much safer than conventionalones.

[0015] In wells with severe loss of circulation, there is no possibilityto detect an influx by observing the return flow rate. Schubert, I. J.and Wright, J. C. in “Early kick detection through liquid levelmonitoring in the wellbore”, IADC/SPE 39400, Dallas, Tex., March 1998propose a method of early detection of a kick through liquid levelmonitoring in the wellbore. Having the wellbore open to atmosphere, hereagain the immediate step after detecting a kick is to close the BOP andcontain the well.

[0016] The excellent review of 800 blow-outs occurred in Alabama, Texas,Louisiana, Mississipi, and offshore in the Gulf of Mexico citedhereinbefore by Skalle, P. and Podio, A. L. in “Trends extracted from800 Gulf Coast blow-outs during 1960-1996” IADC/SPE 39354, Dallas, Tex.,March 1998 shows that the main cause of blow-outs is human error andequipment failure.

[0017] Nowadays, more and more oil exploration and production is movingtowards challenging environments, such as deep and ultra-deepwater.Also, wells are now drilled in areas with increasing environmental andtechnical risks. In this context, one of the big problems today, in manylocations, is the narrow margin between the pore pressure (pressure ofthe fluids—water, gas, or oil—inside the pores of the rock) and thefracture pressure of the formation (pressure that causes the rock tofracture). The well is designed based on these two curves, used todefine the extent of the wellbore that can be left exposed, i.e., notcased off with pipe or other form of isolation, which prevents thedirect transmission of fluid pressure to the formation. The period orinterval between isolation implementation is known as a phase.

[0018] In some situations a collapse pressure (pressure that causes thewellbore wall to fall into the well) curve is the lower limit, ratherthan the pore pressure curve. But, for the sake of simplicity, just thetwo curves should be considered, the pore pressure and fracture pressureone. A phase of the well is defined by the maximum and minimum possiblemud weight, considering the curves mentioned previously and some designcriteria that varies among the operators, such as kick tolerance andtripping margin. In case of a kick of gas, the movement of the gasupward the well causes changes in the bottomhole pressure. Thebottomhole pressure increases when the gas goes up with the well closed.Kick tolerance is the change in this bottomhole pressure for a certainvolume of gas kick taken.

[0019] Tripping margin, on the other hand, is the value that theoperators use to allow for pressure swab when tripping out of the hole,to change a bit, for example. In this situation, a reduction inbottomhole pressure, caused by the upward movement of the drill stringcan lead to an influx.

[0020] According to FIG. 1 attached, based on state-of-the-art designingof wells for drilling, typically a margin of 0.3 pound per gallon (ppg)is added to the pore pressure to allow a safety factor when stoppingcirculation of the fluid and subtracted from the fracture pressure,reducing even more the narrow margin, as shown by the dotted lines.Since the plot shown in FIG. 1 is always referenced to the static mudpressure, the compensation of 0.3 ppg allows for the dynamic effectwhile drilling also. The compensation varies from scenario to scenariobut typically lies between 0.2 and 0.5 ppg.

[0021] From FIG. 1, it can be seen that the last phase of the well canonly have a maximum length of 3,000 ft, since the mud weight at thispoint starts to fracture the rock, causing mud losses. If a lower mudweight is used, a kick will happen at the lower portion of the well. Itis not difficult to imagine the problems created by drilling in a narrowmargin, with the requirement of several casing strings, increasingtremendously the cost of the well. In some critical cases, a differenceas small as 0.2 ppg is found between the pore and fracture pressures.Moreover, the current well design shown in FIG. 1 does not allow toreach the total depth required, since the bit size is continuouslyreduced to install the several casing strings needed. In most of thesewells, drilling is interrupted to check if the well is flowing, andfrequent mud losses are also encountered. In many cases wells need to beabandoned, leaving the operators with huge losses.

[0022] These problems are further compounded and complicated by thedensity variations caused by temperature changes along the wellbore,especially in deepwater wells. This can lead to significant problems,relative to the narrow margin, when wells are shut in to detectkicks/fluid losses. The cooling effect and subsequent density changescan modify the ECD due to the temperature effect on mud viscosity, anddue to the density increase leading to further complications on resumingcirculation. Thus using the conventional method for wells in ultra deepwater is rapidly reaching technical limits.

[0023] On the contrary, in the present application the 0.3 ppg marginsreferred to in FIG. 1 are dispensed with during the planning of the wellsince the actual required values of pore and fracture pressures will bedetermined during drilling. Thus, the phase of the well can be furtherextended and consequently the number of casing strings required isgreatly reduced, with significant savings. If the case of FIG. 1 isconsidered, the illustrated number of casings is 10, while bygraphically applying the method of the invention this number is reducedto 6, according to FIG. 2 attached. This may be readily seen byconsidering only the solid lines of pore and fracture gradient to definethe extent of each phase, rather than the dotted lines denoting thelimits that are in conventional use..

[0024] In order to overcome these problems, the industry has devoted alot of time and resources to develop alternatives. Most of thesealternatives deal with the dual-density concept, which implies avariable pressure profile along the well, making it possible to reducethe number of casing strings required.

[0025] The idea is to have a curved pressure profile, following the porepressure curve. There are two basic options:

[0026] injection of a lower density fluid (oil, gas, liquid with hollowglass spheres) at some point;

[0027] placement of a pump at the bottom of the sea to lift the fluid upto the surface installation.

[0028] There are advantages and disadvantages of each system proposedabove. The industry has mainly taken the direction of the secondalternative, due to arguments that well control and understanding oftwo-phase flow complicates the whole drilling operation with gasinjection.

[0029] Thus, according to the IADC/SPE 59160 paper “Reeled PipeTechnology for Deepwater Drilling Utilizing a Dual Gradient Mud System”,by P. Fontana and G. Sjoberg, it is possible to reduce casing stringsrequired to achieve the final depth of the well by returning thedrilling fluid to the vessel with the use of a subsea pumping system.The combination of seawater gradient at the mud line and drilling fluidin the wellbore results in a bottomhole equivalent density that can beincreased as illustrated in FIG. 2 of the paper. The result is a greaterdepth for each casing string and reduction in total number of casingstrings. It is alleged that larger casing can then be set in theproducing formation and deeper overall well depths can be achieved. Themechanism used to create a dual gradient system is based on a pumplocated at the sea bottom.

[0030] However, there are several technical issues to be overcome withthis option, which will delay field application for some years. The costof such systems is also another negative aspect. Potential problems withsubsea equipment will make any repair or problem turn into a longdown-time for the rig, increasing even further the cost of exploration.

[0031] There are three other main methods of closed system drilling: a)underbalanced flow drilling, which involves flowing fluids from thereservoir continuously into the wellbore is described and documented inthe literature; b) mud-cap drilling, which involves continuous loss ofdrilling fluid to the formation, in which fluid can be overbalanced,balanced or underbalanced is also documented; c) air drilling, where airor other gas phase is used as the drilling fluid. These methods havelimited application, i.e., underbalanced and air drilling are limited toformations with stable wellbores, and there are significant equipmentand procedural limitations in handling produced effluent from thewellbore. The underbalanced method is used for limited sections of thewellbore, typically the reservoir section. This limited applicationmakes it a specialist alternative to conventional drilling under theright conditions and design criteria. Air drilling is limited to dryformations due to its limited capability to handle fluid influxes.Similarly Mud-Cap drilling is limited to specific reservoir sections(typically highly fractured vugular carbonates).

[0032] Thus, the open literature is extremely rich in pointing outmethods for detecting kicks, and then methods for circulating kicks outof the wellbore. Generally all references teach methods that operateunder conventional drilling conditions, that is, with the well beingopen to the atmosphere. However, there is no suggestion nor descriptionof a modified drilling method and system, which, by operating with thewell closed, controlling the flow rates in and out of the wellbore, andadjusting the pressure inside the wellbore as required, causing thatinfluxes (kicks) and fluid losses do not occur or are extremelyminimized, such method and system being described and claimed in thepresent application.

SUMMARY OF THE INVENTION

[0033] In its broadest aspect the present invention is directed to asystem and method of drilling a well by monitoring the flow in and outof the well, as well as monitoring of the flow rates in and out,together with other parameters that produce an early detection of influxor loss independent of the mass flow in and out at that point in time,the well drilled being closed with a pressure containment device at alltimes. Monitoring of flow may be by measurement of mass and/or volumeflow. In a particularly preferred embodiment the system and method ofthe invention comprises monitoring the mass flow in and out of the well.Preferably monitoring is constant throughout a given drilling operation.

[0034] The back pressure in the well is automatically adjusted bypressure/flow control device, controlled by a central control device.This central control device regulates the out flow to keep the flows inand out balanced at all times, or to preemptively adjust thebackpressure to change the ECD (Equivalent Circulating Density)instantaneously in response to an early detection of influx or fluidloss.

[0035] Accordingly the system of the present invention for drilling awell while injecting a drilling fluid through an injection line of saidwell and recovering through a return line of said well where the wellbeing drilled is closed at all times comprises a pressure containmentdevice and pressure/flow control device to a wellbore toestablish/maintain a back pressure on the well, means to monitor thefluid flow in and out, means to monitor flow of any other material inand out, means to monitor parameters affecting the monitored flow valueand means to predict a calculated value of flow out at any given timeand to obtain real time information on discrepancy between predicted andmonitored flow out and converting to a value for adjusting thepressure/flow control device and restoring the predicted flow value.

[0036] In a further aspect the corresponding method of the presentinvention comprises, in relation to the system of the invention ashereinbefore defined, the following steps of injecting drilling fluidthrough said injection line through which said fluid is made to contactsaid means for monitoring flow and recovering drilling fluid throughsaid return line; collecting any other material at the surface;measuring the flow in and out of the well and collecting flow and flowrate signals; measuring parameters affecting the monitored flow valueand means; directing all the collected flow, correction and flow ratesignals to the said central data acquisition and control system;monitoring parameters affecting the monitored flow value and means topredict a calculated value of flow out at any given time and to obtainreal time information on discrepancy between predicted and monitoredflow out and converting to a value for adjusting the pressure/flowcontrol device and restoring the predicted flow value.

[0037] We have found by means of the system and method of the inventionthat the generation of real time metering using a full mass balance andtime compensation as a dynamic predictive tool, which can be compensatedalso for any operational pause in drilling or fluid injection enablesfor the first time an adjustment of fluid injection rate whilecontinuing normal operations. This is in contrast to known open wellmethods which require pausing fluid injection and drilling to unloadexcess fluid, and add additional fluid, by trial and error untilpressure is restored, which can take a matter of hours of fluidcirculation to restore levels. Moreover the system and method providefor the first time a means for immediate restoration of pressure, byvirtue of the use of a closed system whereby addition or unloading offluid immediately affects the well backpressure.

[0038] We have also found that the system and method of the inventionprovide additional advantages in terms of allowing operation with areduced reservoir volume of fluid, by virtue of closed operation underback pressure. Moreover the system and method can be operatedefficiently, without the need for repeated balancing of the system afterany operational pause in drilling, by virtue of the ability tocontinuously circulate fluid even during pauses in drilling, avoidingany undue changes in fluid density and temperature.

[0039] Preferably the system for drilling a well while injecting adrilling fluid through an injection line of said well and recoveringthrough a return line of said well where the well being drilled isclosed at all times comprises:

[0040] a) a pressure containment device;

[0041] b) a pressure/flow control device on the outlet stream;

[0042] c) means for measuring mass and/or volumetric flow and flow rateon the inlet and outlet streams to obtain real time mass or volumetricflow signals;

[0043] d) means for measuring mass and/or volumetric flow and flow rateof any other materials in and out;

[0044] e) means for directing all the flow, pressure and temperaturesignals so obtained to a central data acquisition and control system;and

[0045] g) a central data acquisition and control system programmed witha software that can determine a real time predicted out flow and compareit to the actual out flow estimated from the mass and volumetric flowrate values and other relevant parameters.

[0046] Preferably the means c) for measuring mass flow comprises avolume flow meter and at least one pressure sensor to obtain pressuresignals and at least one temperature sensor to obtain temperaturesignals; and may be a mass flow meter comprising integral pressure andtemperature sensors to compensate for changes in density andtemperature; and the means c) for measuring flow rate comprises meansfor assessing the volume of the hole at any given time, as a dynamicvalue having regard to the continuous drilling of the hole. At least oneadditional pressure and temperature sensor may be provided to monitorother parameters that produce an early detection of influx or lossindependent of the mass flow in and out at that point in time.

[0047] Preferably the means d) comprise means for measuring cuttingsvolume/mass out.

[0048] Most preferably the system comprises:

[0049] a) a pressure containment device;

[0050] b) a pressure/flow control device on the outlet stream;

[0051] c) means for measuring mass flow rate on the inlet and outletstreams;

[0052] d) means for measuring volumetric flow rate on the inlet andoutlet streams;

[0053] e) at least one pressure sensor to obtain pressure data;

[0054] f) at least one temperature sensor to obtain temperature data;

[0055] g) a central data acquisition and control system that sets avalue for an expected out flow and compares it to the actual out flowestimated from data gathered by the mass and volumetric flow rate metersas well as from pressure and temperature data, and in case of adiscrepancy between the expected and actual flow values, adjusting thesaid pressure/flow control device to restore the outflow to the expectedvalue.

[0056] In a further aspect of the invention there is provided a methodfor drilling a well while injecting a drilling fluid through aninjection line of said well and recovering through a return line of saidwell where the well being drilled is closed at all times comprising thefollowing steps:

[0057] a) providing a pressure containment device, suitably of a typethat allows passage of pipe under pressure, to a wellbore;

[0058] b) providing a pressure/flow control device to control the flowout of the well and to keep a back pressure on the well;

[0059] c) providing a central data acquisition and control system andrelated software;

[0060] d) providing mass flow meters in both injection and return lines;

[0061] e) providing flow rate meters in both injection and return lines;

[0062] f) providing at least one pressure sensor;

[0063] g) providing at least one temperature sensor;

[0064] h) injecting drilling fluid through said injection line throughwhich said fluid is made to contact said mass flow meters, said fluidflow meters and said pressure and temperature sensors, and recoveringdrilling fluid through said return line;

[0065] i) collecting drill cuttings at the surface;

[0066] j) measuring the mass flow in and out of the well and collectingmass flow signals;

[0067] k) measuring the fluid flow rates in and out of the well andcollecting fluid flow signals;

[0068] l) measuring pressure and temperature of fluid and collectingpressure and temperature signals;

[0069] m) directing all the collected flow, pressure and temperaturesignals to the said central data acquisition and control system;

[0070] n) the software of the central data acquisition and controlsystem considering, at each time, the predicted flow out of the welltaking into account several parameters;

[0071] o) having the actual and predicted out flows compared and checkedfor any discrepancy, compensated for time lags in between input andoutput;

[0072] p) in case of a discrepancy, having a signal sent by the centraldata acquisition and control system to adjust the pressure/flow controldevice and restore the predicted out flow rate, without interruption ofthe drilling operation.

[0073] Optionally the method comprises additionally providing a means ofmeasurement of drill cuttings rate, mass or volume, when required, tomeasure the rate of cuttings being produced from the well.

[0074] Preferably the system and method comprise additionally means topressurize the wellbore through the annulus, and a step of pressurisingthe wellbore through the annulus, independently of the current fluidinjection path.

[0075] Therefore, the present invention provides a safe method fordrilling wells, since not only is the well being drilled closed at alltimes, but also any fluid loss or influx that occurs is more accuratelyand faster determined and subsequently controlled than instate-of-the-art methods.

[0076] One advantage of the present method over state-of-the-art methodsis that it is able to instantly change the ECD (Equivalent CirculatingDensity) by adjusting the backpressure on the wellbore by closing oropening the pressure/flow control device. In this manner the methodherein described and claimed incorporates early detection methods ofinflux/loss that are existing or yet to be developed as part of themethod herein described and claimed, e.g., tools under development orthat may be developed that can detect trace hydrocarbon influx, smalltemperature variations, pressure pulses etc. The output of these toolsor technology that indicates a kick or fluid loss can be used as afeedback parameter to yield an instant reaction to the detected kick orfluid loss, thus controlling the drilling operation at all times.

[0077] As a consequence, in a patentably distinguishing manner, themethod of the invention allows that drilling operations be carried outin a continuous manner, while in state-of-the-art methods drilling isstopped and mud weight is corrected in a lengthy, time-consuming step,before drilling can be resumed, after a kick or fluid loss is detected.

[0078] This leads to significant time savings as the traditionalapproach to dealing with influxes is very time-consuming: stoppingdrilling, shutting in the well, observing, measuring pressures,circulating out the influx by the accepted methods, and adjusting themud weight. Similarly a loss of drilling fluid to the formation leads toanalogous series of time-consuming events.

[0079] The present invention provides also a method of drilling wherethe bottomhole pressure can be very close to the pore pressure, thusreducing the overbalanced pressure usually applied on the reservoir, andconsequently reducing the risk of fluid losses and subsequentcontamination of the wellbore causing damage, the overall effect beingthat the well productivity is increased.

[0080] The present invention provides further a method to drill with theexact bottomhole pressure needed, with a direct determination of thepore pressure.

[0081] The present invention provides also a method for the directdetermination of the fracture pressure if needed.

[0082] Since both the fracture and pore pressure curves are estimatedand usually are not accurate, the present invention allows a significantreduction of risk by determining either the pore pressure or thefracture pressure, or, in more critical situations, both the pore andfracture pressure curves in a very accurate mode while drilling thewell. Therefore by eliminating uncertainties from pore and fracturepressures and being able to quickly react to correct any undesiredevent, the present method is consequently much safer thanstate-of-the-art drilling methods.

[0083] The present invention provides further a drilling method wherethe elimination of the kick tolerance and tripping margin on the designof the well is made possible, since the pore and fracture pressure willbe determined in real time while drilling the well, and, therefore, nosafety margin or only a small one is necessary when designing the well.The kick tolerance is not needed since there will be no interruption inthe drilling operation to circulate out any gas that might have enteredinto the well. Also, the tripping margin is not necessary because itwill be replaced by the back pressure on the well, adjustedautomatically when stopping circulation.

[0084] By the fast detection of any influx and by having the well closedand under pressure at all times while drilling, the present inventionallows the well control procedure to be much simpler, faster, and safer,since no time is wasted in checking the flow, closing the well,measuring the pressure, changing the mud weight if needed, andcirculating the kick out of the well.

BRIEF DESCRIPTION OF THE DRAWINGS

[0085]FIG. 1 attached is a state-of-the-art log of pore and fracturepressure curves indicated hereinbefore. Included in this figure are thekick tolerance and tripping margin, used for designing the casingsetting points, in this case taken as 0.3 ppg below the fracturepressure and above the pore pressure, respectively. This value iscommonly used in the industry. On the right hand side the number anddiameter of the casing strings required to safely drill this well usingthe current conventional drilling method is shown. As pointed outbefore, the two curves shown are estimated before drilling. Actualvalues might never be determined by the current conventional drillingmethod.

[0086]FIG. 2 attached is a log of the same curves according to theinvention, without the kick tolerance and tripping margin of 0.3 ppgincluded. On the right hand side the number of casing strings requiredcan be seen. With the drilling method described in the presentapplication the elimination of the kick tolerance and tripping margin onthe design of the well is made possible, since the pore and fracturepressure will be determined in real time while drilling the well, withthe well being drilled closed at all times, and, therefore, no safetymargin is necessary when designing the well.

[0087]FIG. 3 attached is a state-of-the-art schematics of thecirculating system of a standard rig, with the return flow open to theatmosphere.

[0088]FIG. 4 attached is a schematic of the circulating system of a rigwith the drilling method described in the application. A pressurecontainment device located at the wellhead, mass flow and fluid flowrate meters on the inlet and outlet streams, pressure and temperaturesensors, cuttings mass/volume measurement device, and other pieces ofequipment have been added to the standard drilling rig configuration.The control system receives all the data gathered and actuates thepressure/flow control device on the outlet stream.

[0089]FIG. 5 attached is a general block diagram of the method describedin the present invention.

[0090]FIG. 6 attached is a flowsheet that schematically illustrates themethod of the invention.

PREFERRED MODE—DETAILED DESCRIPTION

[0091] As pointed out hereinbefore, the present system and method ofdrilling wells is based on a closed-loop system. The inventive methodand system is applied to oil and gas wells, as well as to geothermalwells.

[0092] As regards the mud circuit, the circulation of the drilling fluiddown the wellbore may be through the drill string and the return throughthe annulus, as in state-of-the-art methods, but not limited to it. As amatter of fact, any way of circulation of the drilling fluid may besuccessfully employed in the practice of the present system and method,no matter where the fluids are injected or returned.

[0093] In a very broad way, the system and method of the inventioncomprises adjusting the wellbore pressure with the aid of apressure/flow control device to correct the bottomhole pressure toprevent fluid influx or losses in a pro-active as opposed to thestate-of-the-art reactive manner.

[0094] While several of the devices being described have been used insome configuration or combination, and several of the parametermeasurements have been included in descriptive methods on patents orliterature, none have ever:

[0095] 1. Simultaneously combined the measurement of all criticalparameters to ensure the necessary accuracy required allowing such asystem to effectively function as a whole method.

[0096] 2. Utilized mass flow meters simultaneously on inlet and outletflows.

[0097] 3. Utilized mass measurement of cuttings in conjunction with massflow measurement on inlet and outlet.

[0098] 4. Utilized a pressure/flow control device as an instant controlof ECD during drilling for the purpose of preventing and controllinginflux or losses.

[0099] 5. Defined the use of a pressure/flow control device as apro-active method for adjusting ECD based on early detection ofinflux/loss events.

[0100] The method and system of the invention will now be described inmore detail based on the appended FIGURES.

[0101]FIG. 3 illustrates a drilling method according to state-of-the-arttechniques. Thus, a drilling fluid is injected through the drill string(1), down the wellbore through the bit (2) and up the annulus (3). Atthe surface the fluid that is under atmospheric pressure is directed tothe shale shaker (4) for solid/liquid separation. The liquid is directedto the mud tank (5) from where the mud pumps (6) suck the fluid toinject it through the drill string (1) and close the circuit. In case ofa kick, normally detected by mud tank volume variation indicated bylevel sensors (7), the BOP (8) must be closed to allow kick control. Atthis point the drilling operation is stopped to check pressure andadjust the mud weight to avoid further influxes. Improvements instate-of-the-art drilling methods are generally directed to, forexample, improve the measurement of volume increase or decrease in tank(5). However, such improvements bring only minor changes to the kickdetection procedure; furthermore, no fundamental modifications are knowndirected to the improvement of safety and/or to keeping the drillingmethod continuous, this modification being only brought about by thepresent invention.

[0102] On the contrary, according to FIG. 4 that illustrates oneembodiment of the invention, the drilling fluid is injected through thedrill string (1), going down towards the bottom hole through the bit (2)and up the annulus (3) and is diverted by a pressure containment device(26) through a closed return line (27) under pressure. BOP (8) remainsopen during drilling. The fluid is made to contact pressure andtemperature sensors (9), fluid flow meter (10), mass flow meter (11),flow/pressure control device (12), degasser (13) then to the shaleshaker (4). The shale shaker (4) separates the cuttings (drill solids)from the liquid and the solids have their mass/volume determined (19)while the liquid is directed to the mud tank (5) having the mass/volumedetermined as well (20). All standard drilling parameters are acquiredby a device (21) normally called mud logging. Downhole parameters areacquired by a device (24) located close to the bit (2). The mass/volumeof gas separated in degasser (13) is measured by a device (25).

[0103] The drilling fluid is injected with the aid of pump (6) throughan injection line (14) through which said fluid is made to contact massflow meter (15), fluid flow meter (16), pressure and temperature sensors(17). Devices (7), (9), (10), (11), (15), (16), (17), (19), (20), (21),(24), (25) all acquire data as signals that are directed to a centraldata acquisition and control system (18). System (18) sends a signal tothe pressure/flow control device (12) to open or close it. Whenever itis deemed necessary, a pump (23) may send fluid directly to the annulus(3) through a dedicated injection line (22) via a mass flow meter (28),fluid flow meter (28) and pressure and temperature sensors (28). Forfigure simplification these three devices are shown in just one piece ofequipment. This injection line may be incorporated as part of thestandard circulation system, or embodied in other ways, the purposebeing to provide an independent, of normal drilling circulation, meansof flow into wellbore. The central data acquisition and control system(18) acquires data from device (28).

[0104] According to the concept of the present invention, as illustratedin FIG. 4, a pressure containment device (26) diverts the drilling fluidand keeps it under pressure. Device (26) may be a rotating BOP or arotating control head, but not limited to it. The location of device(26) is not critical. It may be located at the surface or at some pointfurther down e.g. on the sea floor, inside the wellbore, or at any othersuitable location. The drilling fluid is diverted to a closed pipe (27)and then to a surface system. The type and design of the device (26) isnot critical and depends on each well being drilled. It is a standardequipment that is commercially available or readily adapted fromexisting designs.

[0105] As described hereinbefore, upon a signal received from controlsystem (18) the pressure/flow control device (12) opens or closes toallow decrease or increase of the backpressure at the well head so thatthe outflow can be restored to the predicted value determined by system(18). Two or more of these pressure/flow control devices (12) can beinstalled in parallel with isolation valves to allow redundantoperation. Devices (12) can be positioned downstream of the pressurecontainment device (26) at any suitable point in the surface system.Some surface systems may incorporate two or more of such devices (12) atdifferent nodes.

[0106] One critical aspect of the present method is the accuratemeasurement of the injected and returned mass and fluid flow rates. Theequipment used to carry out such measurement is mass flow meters (11,15)and fluid flow meters (10,16). The equipment is installed in theinjected (14) and return (27) fluid lines. These meters may also beinstalled at the gas outlet (25) of the degasser (13) and somewhere (20)on the fluid line between shale shaker (4) and tank (5). Also they maybe installed on the independent injection line (22). The mass and fluidflow meters are commercially available equipment. Multi-phase meters arealso commercially available and may be used. The precision of thisequipment, allows accurate measurement, subsequent control and saferdrilling.

[0107] To further improve the accuracy of the method the cuttingsmass/volume rate can be measured by commercially available equipment(19) to verify that the mass of cuttings being received back at thesurface is correlated with the rate of penetration and wellboregeometry. This data allows correction of the mass flow data and allowsidentification of trouble events.

[0108] The measurements of mass and fluid flow rates provide data thatare collected and directed to a central data acquisition and controlsystem (18).

[0109] The central data acquisition and control system (18) is providedwith a software designed to predict an expected, ideal value for theoutflow, said value being based on calculations taking into accountseveral parameters including but not restricted to rate of penetration,rock and drilling fluid density, well diameter, in and out flow rates,cuttings return rate, bottomhole and wellhead pressures andtemperatures.

[0110] Said software compares the said predicted ideal value with theactual, return flow rate value as measured by the mass flow meters(11,15) and fluid flow meters (10,16). If the comparison yields anydiscrepancy, the software automatically sends a command to apressure/flow control device (12) designed to adjust the return flowrate so as to restore the said return flow rate to the predicted, idealvalue.

[0111] Said software can also receive as input any early detectionparameters available or being developed or capable of being developed.Such input will trigger a chain of investigation of probable scenarios,checking of actual other parameter and any other means (databased orsoftware or mathematical) to ascertain that an influx/loss event hasoccurred. Said software will in such cases pre-emptively adjustbackpressure to immediately control the event.

[0112] Said software will allow for override of the standard detection(state-of-the-art) by the early detection system of the invention andwill compensate and filter for any conflict in fluid/mass flowindication.

[0113] Said software may have filters, databases, historical learningand/or any other mathematical methods, fuzzy logic or other softwaremeans to optimize control of the system.

[0114] The pressure/flow control device (12) used to restore the idealflow is an equipment chosen according to the well parameters such asdiameter of the return line, pressure and flow requirements. Thepressure/flow control device (12) is, as previously stated, standard,commercially available equipment. Alternatively, it may be specificallydesigned for the required purpose.

[0115] According to the present method, the flow rates in and out of thewellbore are controlled, and the pressure inside the wellbore isadjusted by the pressure/flow control device (12) installed on thereturn line (27) or further downstream in the surface system.

[0116] Thus, if the drilling fluid volume returning from the wellbore isincreasing, after compensating for all possible factors it is a signthat an influx is happening. In this case the surface pressure should beincreased to restore the bottomhole pressure in such a way as toovercome the reservoir pressure.

[0117] On the other hand, if the fluid volume returning is decreasing,after compensating for all possible factors it means the pressure insidethe wellbore is higher than the fracture pressure of the rock, or thatthe sealing of the drilling mud is not effective. Therefore, it isnecessary to reduce the wellbore pressure, and the reduction will takeplace by lowering the surface back pressure sufficiently to restore thenormal condition.

[0118] If an early detection signal is confirmed, control system (18)will proactively adjust the backpressure by opening or closingpressure/flow control device (12) to suit the occurred event.

[0119] Thus, upon any undesired event, the system acts in order toadjust the rate of return flow and/or pressure thus increasing ordecreasing the backpressure, while creating the desired conditiondownhole of no inflow from the exposed formation or no loss of fluid tothe same exposed formation. This is coupled with a feedback loop toconstantly monitor the reaction to each action, as well as the necessarysoftware design, and any necessary decision system including but notlimited to databases and fuzzy logic filters to ensure consistentoperation.

[0120] The system and method of drilling oil, gas and geothermal wellsaccording to the present invention is based on the principle of massconservation, a universal law.

[0121] While drilling a well, loss of fluid to the rock or influx fromthe reservoir is common, and should be avoided to eliminate severalproblems. By applying the principle of mass conservation, the differencein mass being injected and returned from the well, compensated forincrease in hole volume, additional mass of rock returning and otherrelevant factors, including but not limited to thermalexpansion/contraction and compressibility changes, is a clear indicationof what is happening downhole.

[0122] Therefore, the expression “mass flow” as used herein means thetotal mass flow being injected and returned, comprised of liquid,solids, and possibly gas.

[0123] In order to increase the accuracy of the method and to expeditedetection of any undesired event, the flow rates in and out of the wellare also monitored at all times. This way, the calculation of thepredicted, ideal return flow of the well can be done with a certainredundancy and the detection of any discrepancy can be made with reducedrisks.

[0124] It should be understood that all the devices used in the presentsystem and method, such as flow metering system, pressure containmentdevice, pressure and temperature sensors, pressure/flow control deviceare commercial devices and as such do not constitute an object of theinvention.

[0125] Further, it is within the scope of the application that anyimprovements in mass/flow rate measurements or any other measuringdevice can be incorporated into the method. Also comprised within thescope of the application are any improvements in the accuracy and timelag to detect influx or fluid losses as well as any improvements in thesystem (18) to manipulate the data and make decisions related to restorethe predicted flow value.

[0126] Thus, improved detection, measurement or actuation tools are allcomprised within the scope of the application.

[0127] It has been shown that measurement of the flow rate only is notaccurate enough to provide a clear indication of losses or gains whiledrilling. That is why the present method envisages the addition of anaccurate mass flow metering (11,15) system that allows the presentdrilling method to be much safer than state-of-the-art drilling methods.

[0128] This mass flow metering principle is extended to include othersubcomponents of the system where accuracy can be improved, such as, butnot limited to measuring the mass flux of cuttings (19) being producedat the shakers (4) and mass outflow of gas (25) from degasser (13), toallow verification and/or improvement of the mass balance beingcontinuously applied to the system.

[0129] Another very important device used in the method and system ofthis invention is the pressure containment equipment (26), to keep thewell flowing under pressure at all times. By controlling the pressureinside the well with a pressure/flow control device (12) on the returnline (27) the bottomhole pressure can be quickly adjusted to the desiredvalue so as to eliminate the losses or gains being detected.

[0130] By having a pressure sensor (24) at the bottom of the string (1)and another one (9) at the surface, the pore and fracture pressures ofthe formations can be directly determined, dramatically improving theaccuracy of such pressure values.

[0131] The assessment of the pore and fracture pressures according tothe method of the invention is carried out the following way: if thecentral data acquisition and control system (18) detects any discrepancyand a decision to actuate the pressure/flow control device (12) is made,it is a sign that either a fluid loss or influx is occurring. TheApplicant has thus ascertained that if there is a fluid loss this meansthat the bottomhole pressure being recorded is equivalent to thefracture pressure of the formation.

[0132] On the contrary, if an influx is detected, this means that thebottomhole pressure being recorded is equivalent to the pore pressure ofthe formation.

[0133] Further, in case of the absence of the pressure sensor in thebottomhole, the variables pore pressure and fracture pressure can beestimated. Thus, the bottomhole pressure is not one of the variablesbeing recorded and only the wellhead or surface pressure is the pressurevariable being acquired. The pore pressure and the fracture pressure canthen be indirectly estimated by adding to the obtained value thehydrostatic head and friction losses within the wellbore.

[0134] The software pertaining to the central data and control system(18) would include all the necessary algorithms, empirical correlationsor other method to allow accurate estimation of the hydrostatic head andfriction losses including any transient effects like, but not limitedto, changing temperature profile along the wellbore.

[0135] Usually, indirect estimation made before drilling, based oncorrelations from logs, or during drilling using drilling parameters arethe best alternatives to determine the pore pressure. Similarly,fracture pressure is also indirectly estimated from logs beforedrilling. In some situations the fracture pressure is determined atcertain points while drilling, usually when a casing shoe is set, notalong the whole well.

[0136] Advantageously, when using the method and system of the inventionthe pore and fracture pressure may be directly determined while drillingthe well. This entails great savings as regards safety and time, twoparameters of utmost importance in drilling operations.

[0137] In state-of-the-art methods, the bottomhole pressure is adjustedby increasing or reducing the mud weight. The increase or reduction inmud weight is most of the time effected based on quasi-empiricalmethods, which by definition implies inaccuracies, which are handled byan iterative process of: —adjusting mud weight, measuring mudweight—this process being repeated until the desired value is reached.To further complicate the matter, due to the time lag, caused by thecirculation time (i.e., time for a full loop movement of a unit elementof mud), the adjustments must be made in stages, e.g., in order toquickly contain an influx, a higher density mud is introduced into thesystem to produce an increase in ECD (Equivalent Circulating Density).At the point where additional hydrostatic head of this higher densitymud, coupled with the hydrostatic head of lower density mud, initiallyin circulation, becomes close to being sufficient to contain the influx,another variation in density of mud must be executed in order not toincrease the ECD to the point of creating losses. This is furthercomplicated by the fact that such density adjustments affect therheology (viscosity, yield point, etc.) of the mud system leading tochanges in the friction component, which in turn has a direct effect onthe ECD. So, in practice, the adjustment of mud weight is not alwayssuccessful in restoring the desired equilibrium of fluid circulation inthe system. Inaccuracy, depending on its extent, may lead to hazardoussituations such as blowouts.

[0138] On the contrary, the method and system of the invention allowsfor a precise adjustment of increase or reduction in bottomholepressure. By using the pressure/flow control device (12) to restore theequilibrium and pressures inside the wellbore, the adjustment is muchfaster achieved, avoiding the hazardous situation of well-known methods.

[0139] It should also be pointed out that in state-of-the-art methodsthe needed bottomhole pressures needed to restore the equilibrium areestimated under static conditions, since these determinations are madewithout fluid circulation. However, the influxes or fluid losses areevents that occur under dynamic conditions. This implies in even moreerrors and inaccuracies.

[0140] Also the speed of adjustment is much greater in the presentmethod, as opposed to the conventional situation, where increasing thedensity (weighting up) or decreasing the density (cutting back) is avery time consuming process. It has been cited before that whiledrilling the ECD is the actual pressure that needs to overcome theformation pressure to avoid influx. However, when the circulation isstopped to make a connection, for example, the friction loss is zero andthus the ECD reduces to the hydrostatic value of the mud weight. Inscenarios of very narrow mud window, the margin can be as low as 0.2ppg. In these cases, it is common to observe influxes when circulationis interrupted, increasing substantially the risks of drilling with theconventional drilling system.

[0141] On the contrary, since the present method operates with the wellclosed at all times which implies a back pressure at all times, thisback pressure may be adjusted to compensate for dynamic friction losseswhen the mud circulation is interrupted, avoiding the influx ofreservoir fluids (kick). Thus the improved safety of the method of theinvention relative to the state-of-the-art drilling methods may beclearly seen.

[0142] Replacement of the dynamic friction loss when the pump stops canbe accomplished by slowly reducing the circulation rate through thenormal flow path and simultaneously closing the pressure flow/contoldevice and trapping a backpressure that compensates for the loss infriction head.

[0143] This same purpose of keeping an unchanging pressure at thebottomhole during circulation stops can be more readily achieved by thefollowing method: the back pressure adjustment can be applied by pumpingfluid, independent of the normal circulating flow path, into thewellbore, to compensate for the loss in friction head, and effecting acontinuous flow that allows easy control of the back pressure byadjustment of the pressure/flow control device (12). This fluid flow maybe achieved completely independent of the normal circulating path bymeans of a mud pump (23) and injection line (22).

[0144] Therefore, a circulation bypass composed of a pump (23) and adedicated injection line (22) to the wellbore annulus allows keeping aconstant pressure downhole during circulation stops and continuouslydetecting any changes in the mass balance indicative of an influx orloss during the circulation stop.

[0145] By using the method and system of the invention, the errors fromestimating the required mud weight based on static conditions areavoided since the measurements are effected under the same dynamicconditions as those when the actual events occur.

[0146] This method also renders possible to run the mud density at avalue slightly lower than that required to balance the formationpressure and using the backpressure on the well to exert an extremelycontrollable ECD at the bottomhole that has the flexibility to beinstantaneously adjusted up or down. This will be the preferred methodin wells with very narrow pore pressure/fracture pressure margins asoccur in some drilling scenarios.

[0147] In this case one of the parameters mentioned in Table 1, which isthe advantage of having three safety barriers is negated. However, thecurrent technical limit on some ultra-deep water wells, due to thenarrow margin, when drilling with the state-of-the art method, leads toa sequence of fluid influxes/losses due to the inaccuracies in manuallycontrolling the mud density and subsequent ECD as described above, thatcan lead to loss of control of the drilling situation and has resultedin the abandonment of such wells due to the safety risks and technicalinability to recover from the situation.

[0148] However, the method of the invention allows, by creating aninstant control mud weight window, controlling the ECD by increasing ordecreasing the backpressure, controlled by the positioning of thepressure/flow control device, to create the conditions for stayingwithin the narrow margin. This results in the technical ability to drillwells in very adverse conditions as in narrow mud weight window, underfull control with the consequent improvement in safety as the well is atall times in a stable circulating condition, while still retaining twobarriers ie. the BOP (blow-out preventer), and the pressure containmentdevice.

[0149] The central data acquisition and control system (18) has a directoutput for actuation of the pressure/flow control device(s) (12)downstream the wellhead opening or closing the flow out of the well torestore the expected value. At this point, if an action is needed, thebottomhole pressure is recorded and associated to the pore or fracturepressure, if a gain or loss is being observed, respectively.

[0150] In case an influx of gas occurs, the circulation of the gas outof the well is immediately effected. By closing the pressure/flowcontrol device (12) to restore the balance of flow and the predictedvalue, the bottomhole pressure reaches back a value that avoids anyfurther influx. At this point no more gas will enter the well and theproblem is limited to circulating out the small amount of gas that mighthave entered the well. Since the well that is being drilled is closed atall times, there is no need to stop circulation, check if the well isflowing, shut-in the BOP, measure the pressures, adjust the mud weight,and then circulate the kick out of the well as in standard methods. Themass flow together with the flow rate measurements provide a veryefficient and fast way of detecting an inflow of gas. Also, the completeremoval of the gas from the well is easily determined by the combinationof the mass flow and flow rates in and out of the well.

[0151] Also the incorporation of early detection of influx/loss devices,which can preemptively result in opening or closing the pressure/flowcontrol device (12), as part of the system, will allow pro-activereaction to influx/losses not achieved by state-of the-art systems.

[0152] The function of the rotating pressure containment device (26) isto allow the drill string (1) to pass through it and rotate, if arotating drilling activity is carried on. Thus, the drill string (1) isstripped through the rotating pressure containment device; the annulusbetween the outside of the drill pipe and the inside of thewellbore/casing/riser is closed by this equipment. The rotating pressurecontainment device (26) can be replaced by a simplified pressurecontainment device such as the stripper(s) (a type of BOP designed toallow continuous passage of non-jointed pipe) on coiled tubingoperations. The return flow of drilling fluid is, therefore, diverted toa closed pipe (27) to the surface treatment package. This surfacepackage should be composed of at least a degasser (13) and shale shaker(4) for solids separation. This way the influxes can be automaticallyhandled.

[0153] In a more appropriate configuration, a closed 3-phase separator(liquid, solid and gas) could be installed replacing the degasser (13).In this case a fully closed system is achieved. This may be desirablewhen dealing with hostile fluids or fluids posing environmentally risks.

[0154] The central data acquisition and control system (18) receives allthe signals of different drilling parameters, including but not limitedto injection and return flow rates, injection and return mass flowrates, back-pressure at the surface, down-hole pressure, cuttings massrates, rate of penetration, mud density, rock lithology, and wellborediameter. It is not necessary to use all these parameters with thedrilling method herein proposed.

[0155] The central data acquisition and control system (18) processesthe signals received and looks for any deviation from expected behavior.If a deviation is detected, the central data acquisition and controlsystem (18) activates the flow pressure/flow control device (12) toadjust the back-pressure on the return line (27). This is coupled with afeedback loop to constantly monitor the reaction to each action, as wellas the necessary software design, and any necessary decision systemincluding but not limited to databases and fuzzy logic filters to ensureconsistent operation.

[0156] In spite of the fact that some early-detection means have beendescribed, it should be understood that the present method and system isnot limited to the described items. Thus, an influx may be detected byother means including but not limited to downhole temperature effects,downhole hydrocarbon detection, pressure changes, pressure pulses; saidsystem pre-emptively adjusting backpressure on the wellbore based oninflux or loss indication before surface system detection.

[0157] The drilling of the well is done with the rotating pressurecontainment device (26) closed against the drill string. If a deviationoutside the predicted values of the return flow and mass flow rates isobserved, the control system (18) sends a signal either to open theflow, reducing the back-pressure or restricting the flow, increasing theback-pressure.

[0158] This deviation may also be a signal from an early detectiondevice.

[0159] The first option (flow opening) is applied in case a fluid lossis detected and the second one (flow restriction), if a fluid gain isobserved. The changes in flow are done in steps previously definedsteps. These step changes can be adjusted as the well is drilled and theeffective pore and fracture pressures are determined.

[0160] The whole drilling operation is continuously monitored so that aswitch to a manual control can be implemented, if anything goes wrong.Any adjustments and modifications can also be implemented as thedrilling progresses. If at all desired, restoring to thestate-of-the-art drilling method is easily done, by not using anymorethe rotating pressure containment device (26) against the drill string(1), allowing the annulus to be open to the atmosphere again.

[0161] A block diagram of the method described in the present inventionis shown in FIG. 5.

[0162] In fact, the present system and method implies many variationsand modifications within its scope and as such it can be applied to allkinds of wells, onshore as well as offshore, and the equipment locationand distribution can vary according to the well, risks, application andrestrictions of each case.

[0163]FIG. 6 is a flowchart illustrating the drilling method of theinvention in a schematic mode, with the decision-making process thatleads to the restoration of the predicted flow as determined by thecentral data acquisition and control system.

[0164] It has been mentioned before that in the conventional drillingmethods the hydrostatic pressure exerted by the mud column isresponsible for keeping the reservoir fluids from flowing into the well.This is called a primary safety barrier. All drilling operations shouldhave two safety barriers, the second one usually being the blow-outpreventer equipment, which can be closed in case an influx occurs. Thedrilling method and system herein described introduces for the firsttime three safety barriers during drilling, these being the drillingfluid, the blow-out preventer equipment, and the rotating pressurecontainment device.

[0165] In underbalanced drilling (UBD) operations, there are just twobarriers, the rotating pressure containment device and the blow-outpreventer, since the drilling fluid inside the wellbore must exert abottomhole pressure smaller than the reservoir pressure to allowproduction while drilling.

[0166] As noted before, there are three other main methods of closedsystem drilling, known as underbalanced drilling (UBD), mud-capdrilling, and air drilling. All three methods have restricted operatingscenarios applicable to small portions of the wellbore, with mud-capdrilling and air drilling only usable under very specific conditions,whereas the method herein described is applicable to the entire lengthof the wellbore.

[0167] TABLE 1 below shows the key differences among the traditionaldrilling system (Conv.), compared with the underbalanced drilling system(UBD) and the present drilling method herein proposed. It can be seenthat the key points addressed by the present application are not coveredor considered by either the traditional conventional drilling system orby the underbalanced drilling method currently used by the industry.TABLE 1 Feature UBD Conv. INVENTION Well closed at all times Yes No YesProduction of reservoir fluids while Yes No No drilling Flow ratesmeasured in and out Yes Yes Yes Mass flow measured in No No Yes Massflow measured out Yes No Yes Pressure/flow control device on the Yes NoYes return line Return flow adjusted automatically No No Yes accordingto mass balance Degasser device on the return line Yes No Yes Kickdetection accurate and fast N/A No Yes Real time¹ kick/loss detectionwhile No No Yes drilling Can instantly utilize input from early N/A NoYes detection of kick/loss Bottom-hole pressure instantly² adjusted NoNo Yes from surface with small action Three safety barriers whiledrilling No No Yes Accurate pore and fracture pressure No No Yesdetermination while drilling Can keep a constant pressure at bottom NoNo Yes hole during connections and trips Immediate control of the wellin case of N/A No Yes kick Can be used to drill the entire well No YesYes Can be used to drill safely within a very No No Yes narrowpore/fracture pressure margin

[0168] The present method is applicable to the whole wellbore from thefirst casing string with a BOP connection, and to any type of well (gas,oil or geothermal), and to any environment (land, offshore, deepoffshore, ultra-deep offshore). It can be implemented and adopted to anyrig or drilling installation that uses the conventional method with veryfew exceptions and limitations.

[0169] Thus the present method can be called INTELLIGENT SAFE DRILLING,since the response to influx or losses is nearly immediate and sosmoothly done that the drilling can go on without any break in thenormal course of action, this representing an unusual and unknownfeature in the technique.

[0170] Therefore, the present system and method of drilling makespossible:

[0171] i) accurate and fast determination of any difference between thein and out flow, detecting any fluid losses or influx;

[0172] ii) easy and fast control of the influx or losses;

[0173] iii) strong increase of drilling operations safety in challengingenvironments, such as when drilling in narrow margin between pore andfracture pressures;

[0174] iv) strong increase of drilling operations safety when drillingin locations with pore pressure uncertainty, such as exploration wells;

[0175] v) strong increase of drilling operations safety when drilling inlocations with high pore pressure;

[0176] vi) easy switch to underbalanced or conventional drilling modes;

[0177] vii) drilling with minimum overbalance, increasing theproductivity of the wells, increasing the rate of penetration and thusreducing the overall drilling time;

[0178] viii) direct determination of both the pore and fracturepressures;

[0179] ix) a large reduction in time and therefore cost spent weighting(increasing density) and cutting back (decreasing density) mud systems;

[0180] x) a large reduction in the cost of wells by reduction in thenumber of necessary casing strings;

[0181] xi) a significant cost reduction in the cost of wells bysignificantly reducing or eliminating completely the time spent on theproblems of differential sticking, lost circulation;

[0182] xii) significantly reducing the risk of underground blow-outs;

[0183] xiii) a significant reduction of risk to personnel compared toconventional drilling due to the fact that the wellbore is closed at alltimes, e.g., exposure to sour gas;

[0184] xiv) a significant cost reduction due to lowering quantities ofmud lost to formations;

[0185] xv) a significant improvement in productivity of producinghorizons by reduction of fluid loss and consequential permeabilityreduction (damage);

[0186] xvi) a significant improvement in exploration success as fluidinvasion due to overweighted mud is limited. Such fluid invasion canmask the presence of hydrocarbons during evaluation by electric logs;

[0187] xv) to drill wells in ultra deep water that are reachingtechnical limit with conventional state-of-the art method;

[0188] xvi) to economically drill ultra-deep wells onshore and offshoreby increasing the reach of casing strings.

I claim:
 1. A system for drilling a well while injecting a drilling fluid through an injection line of said well and recovering through a return line of said well where the well being drilled is closed at all times comprises a pressure containment device and pressure/flow control device to a wellbore to establish/maintain a back pressure on the well, means to monitor the fluid flow in and out, means to monitor flow of any other material in and out, means to monitor parameters affecting the monitored flow value and means to predict a calculated value of flow out at any given time and to obtain real time information on discrepancy between predicted and monitored flow out and converting to a value for adjusting the pressure/flow control device and restoring the predicted flow value.
 2. A system of drilling a well while being drilled with a drill string having a drilling fluid circulated therethrough, while the well is kept closed at all times, wherein the system comprises: a) a pressure containment device; b) a pressure/flow control device on the outlet stream; c) means for measuring mass or volumetric flow and flow rate on the inlet and outlet streams to obtain real time mass or volumetric flow signals; d) means for measuring mass or volumetric flow and flow rate of any other materials in and out; e) means for directing all the flow, pressure and temperature signals so obtained to a central data acquisition and control system; and f) a central data acquisition and control system programmed with a software that can determine a real time predicted out flow and compare it to the actual out flow estimated from the mass and volumetric flow rate values and other relevant parameters.
 3. A system of drilling a well while being drilled with a drill string having a drilling fluid circulated therethrough, while the well is kept closed at all times, wherein said system comprises: a) a pressure containment device; b) a pressure/flow control device on the outlet stream; c) means for measuring mass flow rate on the inlet and outlet streams; d) means for measuring volumetric flow rate on the inlet and outlet streams; e) at least one pressure sensor to obtain pressure data; f) at least one temperature sensor to obtain temperature data; g) a central data acquisition and control system that sets a value for an expected out flow and compares it to the actual out flow estimated from data gathered by the mass and volumetric flow rate meters as well as from pressure and temperature data, and in case of a discrepancy between the expected and actual flow values, adjusting the said pressure/flow control device to restore the outflow to the expected value.
 4. A system according to claim 1 wherein the well is a gas, oil or geothermal well.
 5. A system according to claim 3 wherein the at least one pressure sensor is located at the wellhead.
 6. A system according to claim 3 wherein the system comprises two pressure sensors.
 7. A system according to claim 6, wherein one pressure sensor is at the wellhead and the other one at the bottomhole.
 8. A system according to claim 1 wherein the said central data acquisition and control system is provided with a time-based software to allow for lag time between in and out flux.
 9. A system according to claims 1 or 8 wherein said software is provided with detection filters and/or processing filters to eliminate/reduce false indications on the received mass and fluid flow data, and any other measured or detected parameters.
 10. A system according to claim 1 which comprises three safety barriers, the drilling fluid, the blow-out preventer equipment and the pressure containment device.
 11. Method comprising, in relation to the system according to claim 1, the following steps of injecting drilling fluid through said injection line through which said fluid is made to contact said means for monitoring flow and recovering drilling fluid through said return line; collecting any other material at the surface; measuring the flow in and out of the well and collecting flow and flow rate signals; measuring parameters affecting the monitored flow value and means; directing all the collected flow, correction and flow rate signals to the said central data acquisition and control system; monitoring parameters affecting the monitored flow value and means to predict a calculated value of flow out at any given time and to obtain real time information on discrepancy between predicted and monitored flow out and converting to a value for adjusting the pressure/flow control device and restoring the predicted flow value.
 12. A method of drilling a well while being drilled with a drill string having a drilling fluid circulated therethrough, while the well is kept closed at all times, said method comprising the steps of: a) providing a pressure containment device to the wellbore; b) providing a pressure/flow control device to control the flow out of the well and to keep a back pressure on the well; c) providing a central data acquisition and control system and related software; d) providing mass flow meters in both injection and return lines; e) providing flow rate meters in both injection and return lines; f) providing at least one pressure sensor; g) providing at least one temperature sensor; h) injecting drilling fluid through an injection line through which said fluid is made to contact said mass flow meters, said fluid flow meters, and said pressure and temperature sensors, and recovering drilling fluid through a return line; i) collecting drill cuttings at the surface; j) measuring the mass flow in and out of the well and collecting mass flow signals; k) measuring the fluid flow rates in and out of the well and collecting fluid flow signals; l) measuring pressure and temperature of fluid and collecting pressure and temperature signals; m) directing all the collected flow, pressure and temperature signals to the said central data acquisition and control system; n) the software of the central data acquisition and control system considering, at each time, the predicted flow out of the well; o) having the actual and predicted out flows compared and checked for any discrepancy; p) in case of a discrepancy, having a signal sent by the central data acquisition and control system to adjust the pressure/flow control device and restore the predicted out flow rate, without interruption of the drilling operation.
 13. A method of continuous, safe drilling of a well being drilled with a drill string having a drilling fluid circulated therethrough, while the well is kept closed at all times, said method comprising the steps of: a) providing a pressure containment device to the wellbore; b) providing a pressure/flow control device to control the flow out of the well and to keep a back pressure on the well; c) providing a central data acquisition and control system and related software; d) providing mass flow meters in both injection and return lines; e) providing flow rate meters in both injection and return lines; f) providing at least one pressure sensor to measure pressure; g) providing at least one temperature sensor to measure temperature; h) injecting drilling fluid through an injection line through which said fluid is made to contact said mass flow meters, said fluid flow meters and said pressure and temperature sensors, and recovering drilling fluid through return line; i) collecting drill cuttings at the surface; j)measuring the mass flow in and out of the well and collecting mass flow signals; k) measuring the fluid flow rates in and out of the well and collecting fluid flow signals; l) measuring pressure and temperature of fluid and collecting pressure and temperature signals; m) directing all the collected flow, pressure and temperature signals to the said central data acquisition and control system; n) the software of the central data acquisition and control system considering, at each time, the predicted flow out of the well; o) having the actual and predicted out flows compared and checked for any discrepancy; p) in case of a discrepancy, having a signal sent by the central data acquisition and control system to adjust the pressure/flow control device and restore the predicted out flow rate without interruption of the drilling operation.
 14. A method according to claims 12 or 13 wherein the well is a gas, oil or geothermal well.
 15. A method according to claims 12 or 13 wherein the at least one pressure sensor is located at the wellhead.
 16. A method according to claims 12 or 13 wherein the method comprises two pressure sensors.
 17. A method according to claim 16, wherein one pressure sensor is at the wellhead and the other one is at the bottomhole.
 18. A method according to claims 12 or 13 wherein the Equivalent Circulating Density of the well being drilled is adjusted by closing or opening the pressure/flow control device.
 19. A method according to claims 12 or 13 wherein the discrepancy between actual and predicted out flows is a fluid loss and the adjustment of the pressure/flow control device comprises opening said device to the extent required to counteract fluid loss and reduce backpressure.
 20. A method according to claims 12 or 13 wherein the discrepancy between actual and predicted out flows is a fluid gain and the adjustment of the pressure/flow control device comprises closing said device to the extent required to counteract fluid gain and increase backpressure.
 21. A method according to claims 12 or 13 wherein the predicted ideal value for the outflow is based on calculations taking into account among others rate of penetration, rock and drilling fluid density, well diameter, in and out flow rates, cuttings return rate, bottomhole and wellhead pressures and temperatures.
 22. A method according to claims 12 or 13 wherein the software provided to the central data acquisition and control system receives as input any early detection parameters to ascertain that an influx/loss has occurred.
 23. A method according to claims 12 or 13 wherein the mass flow metering comprises any subcomponents designed to improve accuracy of the measurement.
 24. A method according to claim 23, wherein the subcomponents comprise measuring the mass flux of cuttings being produced at the shakers and mass outflow of gas from the said degasser.
 25. A method according to claims 12 or 13 wherein means are provided to pressurize the well bore through the annulus, independently of the current fluid injection path.
 26. A method according to claim 23, wherein the subcomponents comprise measuring the mass flow and fluid flow into the well bore through the annulus, independently of the current fluid injection path.
 27. A method for the real time determination of the fracture pressure of a well being drilled with a drill string and drilling fluid circulated therethrough, while the well is kept closed at all times, said method comprising the steps of: a) providing a pressure sensor at the bottom of the drill string; b) having fluid and mass flow data generated collected and directed to a central data acquisition and control device that sets an expected value for fluid and mass flow; c) the said central data acquisition and control device continuously comparing the said expected fluid and mass flow to the actual fluid and mass flow; d) in case of a discrepancy between the expected and actual value, the said central data acquisition and control device activating a pressure/flow control device; e) the detected discrepancy being a fluid loss, the value of the fracture pressure being obtained from a direct reading of the bottomhole pressure.
 28. A method for the real-time determination of the pore pressure of a well being drilled with a drill string and drilling fluid circulated therethrough, while the well is kept closed at all times, said method comprising the steps of: a) providing a pressure sensor at the bottom of the drill string; b) having fluid and mass flow data generated collected and directed to a central data acquisition and control device that sets an expected value for fluid and mass flow; c) the said central data acquisition and control device continuously comparing the said expected fluid and mass flow to the actual fluid and mass flow; d) in case of a discrepancy between the expected and actual value, the said central data acquisition and control device activating a pressure/flow control device; e) the detected discrepancy being an influx, the value of the pore pressure being obtained from a direct reading of the bottomhole pressure provided by the said pressure sensor. 